Servo amplifiers are used to drive servo motors in positioning devices of many kinds, from indicating and recording instruments to large steerable antennas and gun mounts. D-c servo motors are now the more widely-used type. Among the numerous general kinds of amplifiers, a common type is a straight d-c amplifier with suitable phase-lead circuits to provide system damping, and output stages with the power capability to drive the motor in either direction.
These and most other prior amplifiers provide a dead band around the null point, where the output is not enough to overcome the mechanical friction in the system; the dead band gets wider as the gain is reduced. In larger systems, the relatively low efficiency of a class-B ouput stage tends to make the heat dissipation and cost undesirably high.
A few prior servo amplifiers have employed pulse modulation means to improve the efficiency, operating the output stages as saturated switches, and to reduce the dead band. For example, Martens et al, U.S. Pat. No. 3,487,458, shows such an amplifier in which the repetition rate of fixed-width pulses is varied proportionally to the magnitude of the error signal. Prior such amplifiers generally vary only one parameter of the pulses, and in linear relation to the input or error signal. They are also relatively complicated, and lack positive circuitry to cut back the motor drive during prolonged extreme overloads, as from a stalled motor or an off-scale input signal.
There is a need for an efficient pulse-type servo amplifier capable of non-linear modulation of both pulse width and repetition rate to provide a minimal dead band, and of simple construction.